A Dosimeter is a device that measures the absorbed dose in tissues (dose equivalents) from a radiation field in rem/n/cm.sup.2. The present invention can be used for measuring energies from thermal to the gigaelectron volts (GeV) neutron range. The invention has numerous applications in nuclear science, high energy physics, reactor facilities, universities, hospitals, factories, environmental agencies, and military, defense and space research projects.
The most commonly used dosimeter for monitoring neutron dose equivalents is the Anderson-Braun (A-B) neutron detector. This type of dosimeter operates in the energy range from 0.025 electron volts (eV) to 14 megaelectron volts (MeV). The Anderson-Braun neutron detector is highly directional and its positioning may result in errors as large as 40% of the measurement.
Birattari et al. discuss some modifications to an A-B neutron detector to extend its range, ("An Extended Range Neutron REM Counter", Nuclear Instruments and Methods in Physics Research, Vol. A297, pp.250-257, 1990). While the modifications provide certain advantages, they do not serve to improve the device's directional sensitivity. This paper talks about a device with an upper limit of 400 MeV and which may become severely inaccurate depending upon its positioning within the radiation field. Finally, the article spells out that "it is apparent that this modified device will not be portable. It can either be thought of as a fixed instrument for ambient monitoring . . . or as a transportable survey meter to be moved around a facility by means of a trolley." Ibid. page 257.
Piesch, U.S. Pat. No. 4,588,898 issued May 13, 1986, uses a spherical moderator sphere as part of a neutron dosimeter. This device uses multiple neutron detectors, all but one of which is placed on the outside of the sphere. The neutron detector in the center of the sphere is not of a spherical shape. The moderating sphere is constructed of polyethylene and therefor its use is limited to moderating low energy neutrons. Polyethylene does not effectively moderate neutrons much above 5 MeV. The Piesch patent teaches the use of multiple detectors (three on the outside of the sphere and one in the center. Finally, Piesch claims an upper range of 10 MeV.
The limitation of current dosimeters, which do not measure neutron dose equivalents above about 15 MeV, is a serious problem at high energy accelerator facilities where monitoring the dose equivalent from highly energetic neutrons is important. Furthermore the directional dependency of most dosimeters may introduce substantial errors into measurements. The present invention represents a substantial advancement over prior radiation measurement art. It can measure dose equivalents above 1 GeV and does not lose accuracy because of positioning. Finally, the present invention is portable.